1. Field of the Invention
The present invention relates to a multi-beam optical system, and more particularly, it relates to a multi-beam optical system for use in an optical system of an image formation such as a multi-color copying machine, color laser printer, or the like.
2. Description of the Related Art
A multi-beam optical system for use on an optical system of an image formation apparatus such as a multi-color copying machine, color laser printer, or the like forms images of plural colors by irradiating respective light beams emitted from plural light sources onto different locations of a beam receiving surface. In accordance with difference in the characteristics of the light beams, it can compose those light beams in the middle portion of optical paths of those light beams and can scan the composed light beams by one scanning means and then separate the scanned light beams according to the differences of the characteristics of those light beams.
Generally in such a multi-beam optical system as mentioned above, there is used a semiconductor laser element as the light source, accordingly, the light beam becomes a laser light. In addition, there are many cases where a polygon mirror is used as the scanning means and a photosensitive drum as the beam receiving surface, respectively.
Now will be described below an example of details of such a conventional multi-beam optical system as mentioned above.
FIG. 1 is a schematic side elevation to illustrate construction of the conventional two-beam optical system.
This two-beam optical system is provided with two semiconductor laser elements 61 and 62 each of which outputs a laser light which is modulated based on the image data obtained by an image reader (not shown), a composing mirror 63, a polygon mirror 65, a lens 69, a photosensitive drum 71, reflecting mirrors 67a, 67b and 67c, a separating mirror 68, and the like.
Each of the semiconductor laser elements 61 and 62 emit a laser light of a different wavelength from each other. In the example of the conventional system, the semiconductor laser element 61 emits a laser light 13a of 810 nm wavelength and the semiconductor laser element 62 emits a laser light 13b of 750 nm of wave-length, respectively. There are arranged, both a semiconductor laser element 61 behind the composing mirror 63 on a straight line connecting the composing mirror 63 and the polygon mirror 65, and the semiconductor laser element 62 in an offset position in front of the composing mirror 63.
The composing mirror 63 consists of a dichroic mirror which transmits a laser light of 780 nm or more wavelength and reflects a laser light of less than 780 nm of wavelength, for example. Accordingly, the laser light 13a emitted from the semiconductor laser element 61 penetrates through the composing mirror 63 and travels toward the polygon mirror 65, and the laser light 13b emitted from the semiconductor laser element 62 is reflected by the composing mirror 63 and travels toward the polygon mirror 65. As a result, both laser light 13a and 13b are composed as one composite laser light 13c and the composite laser light 13c is made to be incident on the polygon mirror 65 from the composing mirror 63.
The composite laser light 13c is deflected by the rotating polygon mirror 65, penetrates through the lens 69 and is reflected by the reflecting mirror 67a, reaching the separating mirror 68.
The separating mirror 68 has the same characterisitic as the above-mentioned composing mirror 63, and it transmits the laser light 13a emitted from the semiconductor laser element 61 out of the composite laser light 13c and reflects the laser light 13b emitted from the semiconductor laser element 62. As a result, the laser light 13a penetrates through the separating mirror 68 and travels a straight optical path, and the laser light 13b is reflected by the separating mirror 68 and travels another optical path different from that of the laser light 13a.
As can be seen from the above description, both of the laser lights 13a and 13b being separated from the composite laser light 13c by the separating mirror 68 travel their respective optical paths and reach different locations on the photosensitive drum 71 which is the beam receiving surface. The laser light 13a is reflected by the reflecting mirror 67b disposed on its optical path and reaches the photosensitive drum 71. On the other hand, the laser light 13b is reflected by the reflecting mirror 67c disposed on its optical path and reaches the photosensitive drum 71.
Both of the laser lights 13a and 13b form electrostatic latent images on the photosensitive drum 71. At this time, in the case where the electrostatic latent image formed by the laser light 13a is developed with a black developer and the electrostatic latent image formed by the laser light 13b is developed with a color developer, such as a red developer, respectively, there can be obtained a multi-color (two colors in this case) hard copy.
Meanwhile, the polygon mirror 65 deflects the composite laser light 13c so that the composite laser light 13c can form a straight scanning line in a direction parallel to the axial direction of the photosensitive drum 71. However, a laser light which penetrates through the lens 69 generally forms an arcing scanning line because of the characteristic and distortion of the lens.
FIG. 2 is a schematic view to illustrate configurations of scanning lines La and Lb to be formed on the photosensitive drum 71 by both laser lights 13a and 13b, respectively.
Assuming that there is formed an upwardly convex arcing scanning line at point that a laser light penetrates through the lens 69, for example, the laser light 13a which was reflected a total of twice by the reflecting mirrors 67a and 67b, than the upwardly convex arcing scanning line La (shown by the broken line in FIG. 2) is formed on the photosensitive drum 71. On the other hand, the laser light 13b which was reflected a total of three times by the reflecting mirror 67a, the separating mirror 68 and the reflecting mirror 67c forms the downwardly convex arcing scanning line Lb (shown by the solid line in FIG. 2) on the photosenstive drum 71. As a result, two scanning lines La and Lb formed on the photosenitive drum 71 by the laser lights lights 13a and 13b, respectively are not a parallel relationship with each other.
In the case where an image of a black line and an image of a red line in a parallel relationship with each other are formed in a multi-color, there will be formed the image of a black line by such an upwardly convex scanning line La of the laser light 13a as shown by the broke line in FIG. 2 and the image of a red line by such a downwardly convex scanning line Lb of the laser light 13b as shown by the solid line in FIG. 2. As a result, there is a difference between a distance in the central portion of the black line image and red line image and a distance in both end portions of these images, and in an extreme case, there is formed images of both the black and red lines being crossed.
As may be clear from the above description, it may be hard to reproduce good images by the conventional multi-beam optical system.